1. Field of the Invention
This invention relates in general to earth boring drill bits and in particular to an improved pressure compensator in an earth boring drill bit.
2. Description of the Prior Art
Earth penetrating tools, including the rotatable cutter-type earth boring drill bit, commonly used lubrication systems that include a pressure compensator to limit the pressure differential between the lubricant and the pressure in the wellbore. A typical type includes a flexible diaphragm located in the lubricant reservoir and separating the lubricant from the borehole fluid. A portion of the diaphragm moves in response to the pressure differential across it tending to equalize the pressure differential between the lubricant reservoir pressure and the borehole fluid pressure.
Temperature increase in the lubricant as the bit is lowered into the well and due to friction heat while rotating causes expansion of the lubricant. Temperature increase may also cause the lubricant to evolve gaseous hydrocarbons. If the drill bit is of the type having positive seals between the cutter and bearing shaft that prevent egress of lubricant, thermal increase results in a pressure build-up in the system that can cause the diaphragm to be damaged as it is pressed against the port leading to the exterior. Excess internal pressure build-up can also damage the seal between the cutter and bearing shaft. A drill bit that contains positive seals is shown in U.S. Pat. No. 3,476,195, while a drill bit having the type of seals that release lubricant at a selected pressure is shown in U.S. Pat. No. 3,230,020.
One device used to reduce the internal pressure build-up in a positively sealed bit is a pressure relief valve, such as shown in U.S. Pat. No. 3,476,195 and in U.S. Pat. No. 3,942,596. Since failure of a pressure relief valve can destroy the effectiveness of the entire lubrication system, it is advantageous to eliminate the valve if internal pressure increases can be minimized by other means.
One way to minimize pressure build-up in a positively sealed bit without a pressure relief valve is to allow room in the reservoir for expansion. Since a relatively high pumping pressure is required to assure complete filling, one means for allowing room for expansion is to remove lubricant after the pressure is released. Another means for allowing expansion space is to use an extra thick compensator cap for filling, replacing it with a thinner cap for use, thereby leaving a void space in the reservoir as shown in U.S. Pat. No. 3,917,028. While these techniques are successful, improvements are desired.